Hi,
I know that AC series is not very "flat", Aurum Cantus recommend and use with Three order crossover for the woofer and the tweeter.

Seas Excel not flat too, the breakup is very difficult for control and is much difficult from all Aurum Cantus AC line.

The most flat woofer with the best breakup are AudioTechnology. I know is the best mid-woofer in my list but is the most expensive here (around 2 time) . First order filter in not problem with AudioTechnology.